1. Field of the Invention
The present invention relates generally to an apparatus for exchanging heat and, more particularly, to such a device which transfers the heat normally lost from a furnace flue for use in heating a room or building structure.
2. Description of the Prior Art
Furnace flues, which exhaust combusted gases from a heating furnace, have long been known to waste heat energy contained within the gases. Technically, the flue exhausts the gases directly from the furnace to a location outside the building so that the poisonous and volatile gases do not become mixed with the ventilating air passing through the building heating system. However, these gases contain a large amount of heat energy which is thus not efficiently used to provide warmth for the ventilating air. Consequently, much of the energy used to create the heat is wasted. Although, a certain amount of heat in the gases passing through the flue heats up the flue pipe, the radiation into the surrounding air from the flue is negligible and thus fails to conserve much of the heat energy passing through the flue.
One previously known device for transferring additional heat from the gases in the flue to the ventilator air is shown in U.S. Pat. No. 4,176,787 to Gary. Gary discloses a flue pipe construction in which a portion of the furnace flue extends across the ventilator duct of the ventilator system. The portion of the flue passing through the cold air return shaft includes internal obstructions which absorb the heat from the gases passing through the flue and thus retain the heat and prolong its contact with the air passing through the ventilator duct. Such a system is disadvantageous in that it obstructs the flow of noxious gases in the flue and increases the risk that these gases will escape through leaks in the ventilation system.
Another known type of device comprises a plurality of air passage tubes which extend through the flue such that the tubes and air passing through them are subjected to the heat energy of the flue gases passing around the tubes. Such a construction also increases obstruction to the flow of the noxious gases flowing through the flue. In addition, since the air flow through the tubes is substantially in a straight line, the air within the tubes is unable to absorb a large amount of heat energy in the short time that the air is passing through the tube.
Another known type of the heat exchange unit is disclosed in U.S. Pat. No. 4,155,505 to Young. Young discloses the use of a plurality of heat exchanging fins secured to the furnace flue and enclosed within an insulated enclosure. A fan within the enclosure causes circulation of the air within the enclosure. The air flow passes over one end of a closed circuit expansion/compression refrigeration system whose hot end extends outside of the enclosure and is disposed within a vented cabinet. A second fan in the vented cabinet causes ventilation air to flow over the hot end of the refrigeration system to transfer the heat energy to the ambient air. Such a unit is disadvantageous in that it is structurely complicated and accordingly very expensive. Since the device involves the use of two fans as well as a compressor and an expander to transfer the heat energy from the enclosure to the vented cabinet, the unit itself requires a large input of energy in order to operate the system. Thus, the device is not only expensive to build but also expensive to operate. Consequently, the system does not efficiently conserve energy.